Method of frequency shifting using a chromium doped laser transmitter

ABSTRACT

A chromium doped solid-state high peak-power laser transmitter emits at a 4861.342 or 4340.50 Angstrom Fraunhofer line, lines of peak blue-seawater transmission and minimum solar radiation. A pink ruby gain medium doped with approximately 0.05% chromium ion is temperature tuned to an R2 line at 6924.51 Angstrom wavelength and an R1 line is dispersively suppressed in a laser oscillator cavity also tuned to 6924.51 Angstrom. The oscillator is Q-switched and the amplified high peak-power 6924.51 Angstrom R2 line output is frequency doubled to 3462.26 Angstrom and hydrogen Raman down-shifted 8310 cm -1  to the second-Stokes at 4861.342 Angstrom to produce said high peak-power output centered on the 4861.342 Angstrom hydrogen-beta Fraunhofer line. Optionally, a temperture tuned 6927.00 Angstrom R2 line laser oscillator wavelength from the pink ruby gain medium doped with approximately 0.05% chromium ion relies on frequency doubling the R2 line at 6927.00 Angstrom to 3462.26 Angstrom and a methane Raman down-shifting of the 3462.26 Angstrom by 5832 cm -1  to the second-Stokes at 4340.50 Angstrom to produce said emission centered on the 4340.5 Angstrom hydrogen Fraunhofer line.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to a copending patent application Ser. No.782,008, pending filed in the Unted States Patent and Trademark Officeon Nov. 16, 1991 by Victor L. Moberg entitled "Method of FrequencyShifting Using chromium Doped Laser Transmitter".

BACKGROUND OF THE INVENTION

Existing laser transmitters for optical communication and rangingapplications operate at wavelengths already occupied by the highbackground noise of the solar spectrum, making reception difficult. Forundersea and sea surface penetrating applications, existing systems usedye lasers with material lifetime problems and expensive secondaryoptical pumping lasers, or xenon chloride excimer lasers with toxic,corrosive gases and 1300 degrees C. lead vapor Raman cells, orsolid-state neodymium based lasers at wavelengths outside the peakseawater transmission band, or low peak power, 1500 degrees C. coppervapor lasers. The excimer/lead Raman system also requires wavelengthcontrol to milli-Angstrom tolerances and must even compensate fordoppler shift due to relative platform motion to be detected by theassociated atomic resonance receivers.

Thus, a need currently exists in the state of the art for an improvementin laser communication and ranging systems using a comparatively simple,solid-state high peak-power laser transmitter operating at a selectwavelength of minimum solar background radiation and maximumblue-seawater transmission.

SUMMARY OF THE INVENTION

A laser transmitter is centered on the low water attenuation and lowbackground noise Fraunhofer line at 4861.342 Angstrom or optionally4340.50 Angstrom, which are wavelengths of minimum solar radiation andhigh blue-ocean transmission. In the 4861.342 Angstrom transmitter anappropriate apparatus and method provides for the temperature tuning ofa pink ruby gain medium doped with approximately 0.05% chromium ion toan R2 line wavelength of 6924.51 Angstrom and dispersively suppressingthe R1 line. The 6924.51 Angstrom emission is Q-switched and amplifiedand frequency doubled to 3462.26 Angstrom. Next, the 3462.26 Angstrom isRaman down-shifted 8310 cm⁻¹ in hydrogen to the second-Stokes to the4861.342 Angstrom hydrogen-beta Fraunhofer line. Optionally, atemperature tuned R2 line at 6927.00 Angstrom wavelength from the pinkruby gain medium doped with approximately 0.05% chromium ion isQ-switched and amplified and frequency doubled to 3463.5 Angstrom. Nextthe 3463.5 Angstrom is Raman down-shifted 5832 cm⁻¹ in methane to thesecond-Stokes to the 4340.50 Angstrom hydrogen Fraunhofer line.

An object of the invention is to provide for improved opticalcommunications and ranging systems operating in sunlight by usingwavelengths at which solar background illumination is minimized.

Another object is to provide an apparatus and method for improvedoptical communications and ranging relying on the hydrogen-betaFraunhofer line at 4861.342 Angstroms, a solar hydrogen absorption lineexhibiting at its cusp less than 15 percent the intensity of the averageblue-green solar background and closely corresponding with the peaktransmission wavelength of seawater, making the line particularly usefulfor undersea or sea-surface penetrating applications.

Another object is to provide an apparatus and method for improvedoptical communications and ranging relying on the hydrogen Fraunhoferline at 4340.50 Angstrom which is a high transmission wavelength ofblue-ocean seawater, making this line particularly useful for underseaor sea-surface penetrating applications.

Yet another object is to provide an apparatus and method for improvedoptical communications and ranging systems relying upon a moderatelycooled solid-state laser, a frequency doubler, and a room temperatureRaman convertor.

Still another object is to provide an apparatus and method for improvedoptical communications and ranging that avoids the criticalmilli-Angstrom wavelength control and exotic atomic resonance receiversheretofore relied upon for the rejection of optical noise by operatingin the multi-Angstrom broad Fraunhofer line regions of minimum solarnoise.

Yet another object of the invention is to provide an apparatus andmethod for improved optical communications and ranging systems which isbased on existing off-the-shelf technology for creating 4861.342 and4340.50 Angstrom Fraunhofer line transmissions.

These and other objects of the invention will become more readilyapparent from the ensuing specification and claims when taken inconjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram representation of the principalconstituents of an apparatus of this inventive concept for transmissionon the 4861.342 Angstrom hydrogen-beta Fraunhofer line.

FIG. 2 shows the wavelengths of the R1 and R2 fluorescent lines of rubyas a function of temperature.

FIG. 3 sets forth the process for the generation of 4861.342 Angstromhydrogen-beta Fraunhofer line radiation.

FIG. 4 shows a block diagram representation of the principalconstituents of an apparatus of this inventive concept for transmissionon the 4340.50 Angstrom hydrogen Fraunhofer line.

FIG. 5 sets forth the process for the generation of 4340.50 Angstromhydrogen Fraunhofer line radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 of the drawings, a solid state laser transmitter10 is capable of transmitting on a hydrogen-beta Fraunhofer line at4861.342 Angstrom or a hydrogen Fraunhofer line at 4340.50 Angstrom whenappropriately tailored in accordance with this inventive concept.Transmitters operating at these wavelengths are of particular interestbecause the daytime background radiation is reduced to less than 15percent or 18 percent, respectively, of the average blue-greenbackground by a solar hydrogen absorption line. A Fraunhofer linetransmitter thus has a daytime signal-to-noise ratio that is about sixtimes higher than an equivalent non-Fraunhofer line laser, an importantfactor for lidar and communications applications. The 4861.342 Angstromwavelength and 4340.50 Angstrom wavelength are also important inundersea or sea-surface penetrating applications because they closelycorrespond with the wavelength of minimum optical attenuation inblue-ocean seawater.

To achieve 4861.342 Angstrom transmission, resonant oscillator R emitsat a controlled ruby R2 line wavelength of 6924.51 Angstrom. Thiswavelength is frequency doubled to the ultraviolet and then Ramandown-shifted in hydrogen to produce the desired 4861.342 Angstromhydrogen-beta Fraunhofer line output. To achieve 4340.50 Angstromtransmission, a controlled 6927.00 Angstrom ruby R2 line is frequencydoubled and then Raman frequency down-shifted in methane to the 4340.50Angstrom hydrogen Fraunhofer line output. Although tens of Joules couldbe produced in the form of pulses hundreds of microseconds long(possibly useful in some limited communications and imagingapplications), generation of lower energy pulses with higher peak-powersby Q-switched systems is selectable.

Resonant oscillator R includes an unclad or sapphire clad pink ruby gainmedium or element 11 doped with approximately 0.05% chromium ion, which,typically, exhibits a few tenths of a percent overall electrical tooptical conversion efficiency when flashlamp pumped. The pink ruby gainelement has been shown to have advantages over the more efficientfour-level Nd:YAG system in being able to store approximately 30 timesthe energy per cubic centimeter and having up to a 90 percent quantumyield, but disadvantages in requiring higher pump thresholds and moreefficient cooling for a given repetition rate. Ruby is similar toneodymium doped glass in laser characteristics and applications, but hasnot yet enjoyed a sustained research effort to improve its average powerlevels. Conceivably, some of the same techniques which have pushedNeodymium doped glass lasers to the 1000 watt level could be applied toruby, which actually has a better thermal conductivity.

Ruby lasers have been commercially available with multi-mode long-pulse(hundreds of microseconds) energy levels in the low hundreds of Joulessince the 1960's. Q-switched output is typically 15 percent of thelong-pulse energy. Commercial Q-switched systems have been availablewith up to 20 Joules TEMe₀₀ output in 25 nanosecond pulse widths at lowpulse rates. Adding a Q-switch, temperature tuning, and R2 lineselection to a commercially available 15 Joule long-pulse ruby system(for example an Advanced Laser Systems Inc. Model 604) would providemulti-Joule TEM₀₀ pulses of 10 to 20 nanoseconds duration at about 5 ppsrate. Frequency doubling with 60% conversion efficiency and Ramanshifting to the second-Stokes with 35% conversion efficiency wouldresult in approximately 500 mJ pulses on the desired Fraunhofer linewavelength. Considerably lower (by a factor of 10 to 1000) pulseenergies are required for most undersea communications and rangingapplications at these wavelengths. Holding average output powerconstant, lower pulse energies would yield correspondingly highertransmission rates from the same system. The capabilities provided forundersea communications and ranging in accordance with this inventiveconcept are significant.

Ruby gain element 11 doped with approximately 0.05% chromium iongenerates at least a chromium R2 line fluorescence. A typical such rubygain element is available from General Ruby and Sapphire Corp. of NewPort Richey, Fla., as a Series 10R Czochralski laser rod. Alternatively,a ruby gain element complete with R2 line resonator cavity and pumpingsource is available from R-K Manufacturing Co. of Hollywood, Fla., as acustom version of the model 6000R-1 laser.

Gain element 11 converts broadband non-coherent optical pump radiation12° emitted from a pumping radiation source 12 into narrow-bandfluorescent emissions 11, which include the R2 line wavelength andamplifies such emissions propagating in laser resonator R through theprocess of stimulated emission. Gain element 11 is shaped to permituniform thermal regulation to within, for ruby, ±1.4 degrees C. of theset point by an active cooling/temperature tuning system 13. The uniformthermal regulation is aided in part by the use of slab, plate, thin rodor deeply grooved or hollow rod geometry of gain element 11 of resonatorR. Divergence of beam 11° is minimized by suitable curvature of outputfaces of gain element 11 to compensate for thermal effects at theoperating temperature.

Pumping radiation source 12, either continuous or pulsed, provides asufficient emission 12^(o) in the useful laser gain medium pumpabsorption bands, 3600 Angstrom to 4500 Angstrom and 5100 Angstrom to6000 Angstrom for ruby, to initiate the desired lasing at the R2 line atthe operating temperature. Diode pumping is theoretically possible withthe recently developed orange, green, and blue LEDs and laser diodes,but not yet practical due to efficiency and lifetime limitations ofthese devices. Until visible emission diode technology matures, othermore conventional sources, such as flash and continuous arc lamps, maybe selected providing that these sources have the appropriate emissions.Xenon Corp. of Woburn, Mass., provides laser pump flash lamps modelsN-186, N-187 and N-189 suitable for ruby lasers. Alternatively, a flashlamp pumping source is available from R-K Manufacturing Co. ofHollywood, Fla., integrated into an R2 line resonator cavity completewith ruby gain element, as a custom version of the model 6000R-1 laser.Irrespective what particular pumping sources are selected, care must betaken that the chosen optical pump source provides the proper pumpingwavelengths that the gain medium 11 of laser resonator R converts to theappropriate laser emission 11^(o).

Thermal loading and pump-induced optical damage or degradation of thegain medium or element 11 are minimized by appropriate opticalfiltration, not shown to avoid belaboring the obvious, of pump light12^(o) at pumping radiation source 12 to remove wavelengths outside theuseful excitation bands. When pulsed pumping of the pulse source isdesirable, an appropriate timing of a Q-switch 14 in synchronizationwith the pulsed pumping is preferred to provide maximum gain at themoment of Q-switch opening. The same optical source 12 may be used topump both the laser oscillator gain element 11 and an amplifier gainmedia 16, to be described below, note the dotted line connecting theseelements in FIG. 1, although a combination of separate, closelysynchronized pulse or continuous sources may be chosen to pump thesecomponents in some situations. Separate pump sources and cavities willgenerally be preferred for high energy applications.

A temperature tuning means 13 is operatively associated with gainelement 11 of laser resonator R and employs an active feedbackcontrolled cooling system for maintaining the R2 line fluorescence peak11, of the gain medium at the desired wavelength by thermal regulationof gain element 11. Such a system is provided by FTS Systems, Inc. ofStone Ridge, N.Y., as the Series RC-211-ULT recirculating cooler,capable of +/-0.1 degree C. temperature control over a range from -80degrees C. to +40 degrees C.

The desired oscillator wavelength 11^(o), i. e. emission wavelength ofresonator R, is dependent upon the transmitter wavelength 18^(o) andRaman conversion means 17, to be described below, employed and isdetermined by the Equation (1) for the wavelength in Angstroms: ##EQU1##where v is the inverse centimeter wavenumber expression for thetransmitter wavelength (20570.46 cm⁻¹ for 4861.342 Angstroms), n is theindex of refraction of air which equals 1.0002926, N is the integerspecifying the Raman shift Stokes number required, and S is the Ramanshift in inverse centimeters of the Raman medium. The oscillatorwavelength must fall within the temperature tuning range of the lasergain medium, and the Stokes number should be kept low (three or less)for maximum conversion efficiency. For a 4861.342 Angstrom transmitterembodiment of this inventive concept using a natural hydrogen Ramanshift (4155 inverse centimeters) to the second-Stokes, the requiredwavelength is: ##EQU2##

Considering the 4861.342 Angstrom transmitter embodiment of thisinventive concept, the thermal regulating system 13 thusly temperatureshifts the R2 wavelength from the room temperature value, 6929 Angstromfor ruby at 25 degrees C., to the desired 6924.51 Angstrom for ruby atapproximately -36 degrees C. (±1.4 degrees C.) and maintains it theredespite thermal loading from optical pump 12. Looking to FIG. 2, whichshows the wavelengths of the R1 and R2 fluorescent lines of anappropriately pumped ruby gain element as a function of temperaturechanges, the temperature of the ruby gain element must be maintained bythermal regulating system 13 to within ±1.4 degrees C. of approximately-36 degrees C. to assure the desired 6924.51 Angstrom R2 line emission.

This precise operating temperature is dependent upon the gain mediumtemperature/wavelength coefficient. Heat is removed from ruby gainelement 11 and the temperature maintained by such means as contact witha thermally controlled liquid or gaseous coolant or by conductionthrough a cold-finger or heat-pipe to a heat pump. A Fluorinert® FC-84liquid coolant based system is available from FTS Systems, Inc. of StoneRidge, N.Y., as the Series RC-211-ULT recirculating cooler, capable of+/-0.1 degree C. temperature control over a range from -80 degrees C. to+40 degrees C. Amplifier gain media 16, mentioned above and elaboratedon below, optionally, may share thermal regulating system 13 of laseroscillator 11 if both gain media have the same temperature/wavelengthcoefficient, note the dotted line connecting these elements in FIG. 1.Otherwise, power amplifier 16 and laser oscillator 11 may be separatelytemperature tuned to assure the same R2 line wavelength.

A Q-switch 14 functions as an optical switching means to spoil resonatorQ during buildup of gain medium population inversion and to quicklypermit high resonator Q at the desired time of laser emission of the R2line. The Q-switch permits the gain medium to store a maximum of pumpenergy before releasing it in a very brief burst with very high peakpower. The Q-switch may function as the modulating element for the highpeak-power laser ranging and digital communication system when such isthe intended use of the system. Q-switches typically employ saturableabsorber, electro-optic, acousto-optic, or mechanically rotating opticalmeans. As mentioned above, for pulsed pump sources, Q-switch 14 andpumping source 12 are synchronized to provide maximum oscillator peakpower output. Typical Q-switches which could be selected would be theCleveland Crystals, Inc. of Cleveland, Ohio, Series QX1020 or QX1630Pockels cells. Alternatively, a Q-switch is provided by R-KManufacturing Co. of Hollywood, Fla., as an option for the model 6000R-1ruby laser, a custom version of which lases on the R2 line.

Resonator R also includes a totally reflecting element 15a which mayinclude a mirror, and a partially reflecting element 15b, which mayinclude a partially reflective mirror, appropriately spaced apart todefine a required optical resonator laser cavity. The optical resonatorcavity is selected to provide a means of narrow-line laser wavelengthselection and includes appropriate dispersive elements to suppress thedominant R1 emission line (6939.5 Angstrom at -36 degrees C. for a rubygain element), in preference for the R2 line emission of 6924.51Angstrom at the operating temperature. The optical resonator permits thebuildup of oscillations at the R2 line, which are amplified during eachpass through the gain medium. Typical examples of such components couldbe the Spindler & Hoyer, Inc. of Milford, Mass., part number 336690quartz dispersion prism and the Melles Griot, Inc. of Irvine, Calif.,product number 08MLQ001/322 totally (99.5%) reflecting mirror andproduct number 08COB007 partially (70%) reflecting mirror.Alternatively, a R2 line tuned resonator cavity complete with ruby gainelement and pumping source is available from R-K Manufacturing Co. ofHollywood, Fla., as a custom version of the model 6000R-1 laser. Theresonator is tuned for narrow line operation within ±0.1 Angstrom of thewavelength determined by Equation (1) above by such means as anadjustable etalon, narrow-band mirror, narrow-band filter, prism,diffraction grating, or other suitable narrow-band, low in-band lossdispersive elements associated with the totally reflecting element and apartially reflecting element. Locking of the laser oscillator wavelengthto 6924.51 Angstrom may also be facilitated by the injection of seedradiation of 6924.51 Angstrom wavelength into the resonator cavity froman external low power source such as an optical parametric oscillator ordiode laser. A laser resonator favoring low or single mode, lowdivergence output beams is preferred, as high beam quality enhancesfrequency doubler and Raman cell conversion efficiency.

A power amplifier 16 is included to receive beam 11^(o) to serve as ameans of laser beam amplification which is matched in R2 line wavelengthand synchronized in timing with the laser resonator R made up ofelements 11 through 15. Power amplifier 16 includes a chromium dopedoptical gain medium 16.1 of substantially the same consistency as thatof the gain element 11, an optical pumping means 16.2 which can be asharing of the pumping emissions of pumping source 12, or a separatepumping means could be provided (not shown in the drawings) if thisadditional structure is found to be more expedient, and a regulatedtemperature tuning means 16.3 which could be a sharing of temperaturetuning means 13 (see the dotted line in FIG. 1) or a separate such unitif so desired. Power amplifier 16 therefore, typically may be the sameas gain medium 11, pump source 12 and temperature control means 13 asdescribed above. The power amplifier 16 increases the 6924.51 Angstrombeam 11^(o) to a high peak power 11^(oo) while substantially maintainingthe low beam divergence and narrow-line operation of laser resonator R.The optical power amplifier may be omitted for some applications whichrequire less peak-power, provided that frequency doubler 17 and Ramanconverter 18 thresholds are met.

A crystalline non-linear optical element 17 is appropriately disposed toreceive the amplified 6924.51 Angstrom beam 11^(oo) and is included tofunction as an optical frequency doubling means. This crystallinenon-linear optical element, or doubler, converts the 6924.51 Angstromoutput 11^(oo) to a 3462.26 Angstrom output 17^(o). The doubler has goodtransmission at both the 6924.51 Angstrom and the 3462.26 Angstromwavelengths and is able to phase match to the incoming 6924.51 Angstrombeam. A typical doubler which could be included as the crystallinenon-linear optical element 17 is Lithium Triborate, LiB₃ O₅ ("LBO")provided by Quantum Technology, Inc. of Lake Mary, Fla.

A hydrogen Raman cell converter 18 receives the 3462.26 Angstrom beam17^(o) from crystalline non-linear optical doubler element 17 to serveas a means of 8310 cm⁻¹ Raman down-shifting to 4861.342 Angstrom, thesecond-Stokes wavelength in hydrogen and the hydrogen-beta Fraunhoferline. This converter utilizes stimulated scattering in a Raman mediumsuch as hydrogen to increase the wavelength, or decrease the wavenumber,of the incoming laser beam 17^(o) to a Raman down-shifted 4861.342Angstrom second-Stokes wavelength 18^(o). A Raman converter is typicallyeither a single-pass device, a multi-pass device, or a Ramanoscillator/amplifier combination of single or multi-pass devices. Atypical hydrogen filled Raman converter is available from PhotonInteractions of Alexandria, Va. as one Raman material option of theRS-100 series Raman cells.

The Raman medium pressure or concentration, composition, beam focusingoptics and path length of hydrogen Raman cell converter 18 are optimizedin accordance with techniques freely practiced by one skilled in the artto which this invention pertains for conversion to the desired Stokesshift. For efficient Raman shifting of 3462.26 Angstrom 17^(o) to thesecond-Stokes wavelength 18^(o) which is 4861.342 Angstrom in naturalhydrogen, these parameters are adjusted to promote the four-wave mixingwhich leads to conversion of first-Stokes to second-Stokes radiation.Exemplary techniques for efficient hydrogen Raman conversion to thesecond Stokes are disclosed by A. Luches et al., Applied Physics B, Vol.No. 47 (1988) page 101, and V. Baranov et al., Soviet Journal of QuantumElectronics, Vol. 18 No. 10 (1988) page 1272. For a hydrogen Ramanconverter such as that employed in this inventive concept, a relativelylong Raman oscillator operating above threshold power levels withapproximately 10 atmospheres hydrogen pressure and a short focal lengthbeam reduction optic would tend to promote second-Stokes output. Thislow energy 4861.342 Angstrom output would then be filtered to removevestigial lower and higher Stokes components, and used to seed anapproximately 40 atmosphere pressure hydrogen Raman amplifier as theamplifier receives the suitably delayed main portion of the 3462.26Angstrom laser beam. Optionally, a separate external source of 4861.342Angstrom radiation, such as a blue-emitting diode laser or opticalparametric oscillator, may be employed to provide the Raman amplifierseed radiation.

A transmitter fabricated in accordance with this inventive concept andemploying continuous optical pumping from pumping sources 12 willgenerally have lower peak-power than that obtainable with a highpeak-power pulsed pump source. The lower peak-power beam may requirelower threshold Raman converters, such as those based on smallcross-section waveguide or capillary structures or multi-pass resonatorstructures. Single mode, low divergence beams are required for highconversion efficiency with these Raman converters.

The 6924.51 Angstrom laser oscillator wavelength is applicable tooperation with a natural hydrogen Raman cell at two Stokes shifts. OtherRaman media have different degrees of shift and could utilize the sameor other Stokes orders to meet the requirements of equation (1) for awavelength within the temperature and resonator tuning capability of thelaser oscillator.

Operationally, referring to FIG. 3, the 4861.342 Angstrom chromium dopedlaser transmitter calls for pumping A and temperature tuning B of gainelement 11 of laser resonator R (elements 11-12-13-14-15) to emit the R2line 11^(o) at 6924.51±0.1 Angstrom. While temperature tuning B, rubygain medium 11 is maintained at approximately -36 degrees C. by thetemperature tuning means 13. The 6949 Angstrom R1 line temperatureshifts to 6939.5 Angstrom, which is suppressed by the appropriateconfiguration of dispersive resonator elements 15a and 15b inconjunction with gain element 11. The oscillator cavity defined byelements 11, 15a and 15b is dispersively tuned C to 6924.51±0.1 Angstromto assure that 6924.51 Angstrom 11^(o) is the emission of laseroscillator resonator R. Laser oscillator Q-switch 14 is appropriatelytriggered so that an output beam 11^(o) at 6924.51 Angstrom is fed topower amplifier 16 which produces a high peak-power 6924.51 Angstromoutput 11^(oo) for input to frequency doubler 17. Crystalline non-linearoptical frequency doubling element 17 effects a frequency doubling D ofthe amplified 6924.51 Angstrom output 11^(oo) to a 3462.26 Angstromoutput 17^(o). The Raman cell receives the 3462.26 Angstrom 17^(o) andeffects a Raman down-shifting E of the 3462.26 Angstrom 17^(o) by 8310cm⁻¹ to the second-Stokes wavelength 18^(o) which is the low noiseFraunhofer line at 4861.342 Angstrom.

Contemporary optical communication and ranging systems operating insunlight are improved in performance by utilizing wavelengths at whichthe solar background illumination is minimized. The invention providessuch wavelength transmissions on the hydrogen-beta Fraunhofer line at4861.342 Angstrom, a solar hydrogen absorption line exhibiting, at itscusp, solar noise less than 15 percent the intensity of the averageblue-green solar background. This line also closely corresponds with thepeak transmission wavelength of blue-ocean seawater, making itparticularly useful for undersea or sea-surface penetratingapplications. This inventive concept utilizes only a modestly cooledsolid-state laser, a frequency doubler, and a room temperature Ramanconverter. It reduces the complexity of contemporary transmitters ofthis type by not requiring critical milli-Angstrom wavelength controland exotic atomic resonance receivers to reject solar noise as itoperates in a multi-Angstrom broad region of minimum solar radiation.

The above mentioned configuration can be modified within the teachingsof this inventive concept to accommodate other component architectures.For example, this inventive concept can be modified to transmit at a4340.50 Angstrom hydrogen Fraunhofer line, see the transmitter 10' inFIG. 4. The Fraunhofer line at 4340.50 Angstrom also is a solar hydrogenabsorption line exhibiting at its cusp less than 18 percent of theintensity of average blue solar background. This line is also anexcellent transmission wavelength of blue-ocean seawater, making itparticularly useful for undersea or sea-surface penetratingapplications. A high peak-power laser transmitter with output wavelengthcentered at 4340.50 Angstrom may be required for practical opticalcommunications and ranging systems making use of this hydrogenFraunhofer line.

As with the embodiment described above, this arrangement utilizes aseries of interrelated components and processes, each individuallycommon in the art but novel in their combination to produce a highpeak-powered laser transmitter system emitting at the center of the4340.50 Angstrom hydrogen Fraunhofer line. A 4340.50 Angstrom hydrogenFraunhofer line transmitter 10' has an optical resonator R' containinglaser gain element or medium 11, consisting of unclad or sapphire cladpink ruby, chromium ion doped to approximately 0.05 percent. The rubygain element is appropriately pumped by a pumping source 12' to emit atleast an R2 line at 6927.00 Angstrom with the ruby gain element held ata temperature of approximately 8.13 degrees C., which is established bya temperature stabilizing means 13'.

The ruby gain element is shaped appropriately to permit uniform thermalregulation to within ±1.4 degrees C. of approximately +8.13 degrees C.by active cooling/temperature tuning system 13'. This is facilitatedthrough use of slab, plate, or deeply grooved or hollow rod structures.Divergence of beam 11^(o') is minimized by suitable curvature of outputfaces of gain element 11' to compensate for thermal effects at theoperating temperature.

An optical pumping source 12', either continuous or pulsed, withsufficient emission in the proper pumping bands of the ruby gain mediumstimulates the medium to emit the R2 line at 6927.00 Angstrom atapproximately +8.13 degrees C. Thermal loading and ultraviolet damage tothe ruby gain element are minimized by optical filtration of the pumplight from source 12' to remove wavelengths outside the useful rubyabsorption bands. Pulsed pump source timing is synchronized with thefiring of a Q-switch 14, to provide maximum gain at the moment ofQ-switch opening.

An active cooling/temperature tuning system 13' effects a temperaturetuning and the maintaining of the peak of the ruby R2 gain curve within±0.1 Angstrom at 6927.00 Angstrom by thermal regulation of the ruby gainmedium. Because of the temperature/wavelength coefficient of the ruby,the ruby temperature is maintained to within ±1.4 degrees C. ofapproximately +8.13 degrees C.

An optical resonator laser cavity of resonator R' is created to includea totally reflective element 15a' which may include a mirror and apartially reflective element 15b' which may include a partiallyreflective mirror with wavelength selective features to suppress thedominant ruby gain element R1 emission line of 6941 Angstrom (at +8.13degrees C.) in preference for the R2 emission line of 6927.00 Angstrom.The resonator is tuned for single wavelength operation at 6927.00Angstrom by an adjustable etalon, filter, prism, grating, or othersuitable wavelength selective element which may be included in thetotally reflective element and the partially reflective element.Wavelength locking may also be facilitated by the injection of seedradiation of 6927.00 Angstrom wavelength into the resonator cavity froman external low power source such as an optical parametric oscillator ordiode laser.

Optical switch 14' functions to spoil resonator Q during buildup ofpopulation inversion and quickly permit high resonator Q at the desiredtime of laser emission similarly to the first described embodiment andmay be of a passive chemical, electro-optic, acousto-optic, ormechanical nature. The Q-switch is synchronized with pulsed pumpingsource 12' to provide maximum peak power output.

A ruby power amplifier 16', if used, is matched in wavelength andsynchronized in timing with the resonator R, and consists of elements16.1', 16.2' and 16.3' which are substantially the same as elements 11',12' and 13', respectively. Power amplifier 16' provides increased peakpower while maintaining the low beam divergence and narrow lineoperation of the lower power laser oscillator R'. The power amplifierwould be located between the output 11^(o') of the laser oscillator R,and the input 11^(oo') of a frequency doubler 17' as shown in FIG. 4.

A crystalline non-linear optic element of a frequency doubler 17'receives the amplified 6927.00 Angstrom beam 11^(oo') and functions asan optical frequency doubling means. The crystalline non-linear opticalelement frequency doubles the amplified 6927.00 Angstrom output 11^(oo')to a 3463.5 Angstrom output 17^(o'). The frequency doubling element ischosen for low doubling threshold, high damage threshold, goodtransmission at both the 6927.00 Angstrom fundamental and 3463.5Angstrom second harmonic wavelengths and the ability to phase match tothe incoming 6927.00 Angstrom 11^(oo'). A typical doubler which could beincluded as the crystalline non-linear optical element 17' is LithiumTriborate, LiB₃ O₅ ("LBO") provided by Quantum Technology, Inc. of LakeMary, Fla.

A methane Raman converter cell 18, receives the 3463.5 Angstrom beam17^(o') from crystalline non-linear optical doubler element 17' tofunction as a means of Raman down-shifting by 5832 cm⁻¹ to thesecond-Stokes 4340.50 Angstrom hydrogen Fraunhofer line output 18^(o').The Raman converter cell utilizes stimulated scattering in a Ramanmedium such as methane pressurized to several atmospheres to increasethe wavelength, or decrease the wavenumber, of the incoming laser beamto a 4340.50 Angstrom output 18^(o') centered on the 4340.50 Angstromhydrogen Fraunhofer line. The Raman shifter is typically either asingle-pass device, a cascade of single-pass devices, or a deviceutilizing an optical resonator to reduce physical length. A typicalmethane filled Raman converter is available from Photon Interactions ofAlexandria, Va. as one Raman material option of the RS-100 series Ramancells. Efficient wavelength conversion is promoted by the use ofco-propagating seed radiation of the desired 4340.50 Angstromwavelength, either from an external source such as an optical parametricoscillator or blue-emitting diode laser, or more conveniently from anintegral low power Raman oscillator receiving a fraction of the inputradiation. The pressure, gas mixture, beam focusing, and path length ofmethane Raman cell 18, are optimized in accordance with techniquesfreely practiced by one skilled in the art to which this inventionpertains for conversion to the second-Stokes shift. For efficient Ramanshifting of 3463.5 Angstrom 17^(o') by 5832 cm⁻¹ to the second-Stokeswavelength 18^(o') which is 4340.50 in methane, these parameters areadjusted to promote four-wave mixing which converts first-Stokesradiation to the second-Stokes. For a Raman converter such as thatemployed in this inventive concept, a relatively long Raman oscillatoroperating well above threshold with a few atmospheres pressure ofmethane and a short focal length beam reduction optic would tend topromote second-Stokes output. This output would then be filtered toremove vestigial lower and higher Stokes components, and used to seed anapproximately 60 atmosphere pressure methane Raman amplifier as theamplifier receives the suitably delayed main portion of the 3463.5Angstrom beam 17^(o'). Raman cell window materials are selected formechanical strength, high optical damage threshold, and goodtransmission at the appropriate input or output wavelength. An opticalisolator, such as the Faraday rotators provided by Optics for Researchof Caldwell, NY, is recommended in the beam path before the methaneRaman cell to prevent damage to the components of the laser amplifier oroscillator or frequency doubler by radiation back-scattering from themethane Raman medium.

Referring to FIG. 5, in operation this embodiment has a pumping A' of again element 11' and a temperature tuning B' to an R2 line 6927.00Angstrom wavelength from gain element 11' and a dispersive opticalresonator tuning C' to an R2 line 6927.00 Angstrom wavelength from laseroscillator R' and a frequency doubling D' of the 6927.00 Angstrom to3463.5 Angstrom. Next a methane Raman shifting E' of the 3463.5 Angstromby 5832 cm⁻¹ to the second-Stokes wavelength at 4340.50 Angstrom iseffected to provide the desired 4340.50 Angstrom hydrogen Fraunhoferline output.

By operating on a Fraunhofer line, solar background noise can be reducedby approximately 82 percent. The blue wavelength is also excellent forundersea and sea-penetrating applications because of high blue-oceanoptical transmission.

Obviously, many modifications and variations of the present inventionare possible if the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than specifically described.

I claim:
 1. A method of emitting at a 4861.342 Angstrom hydrogen-betaFraunhofer line of peak blue-seawater transmission and minimum solarradiation comprising:pumping a pink ruby gain element doped withapproximately 0.05% chromium ion with a pumping source means to emit atleast an R2 line; temperature tuning said pink ruby gain element dopedwith approximately 0.05% chromium ion to approximately -36° C. to emitat said R2 line at 6924.51 Angstrom wavelength with a temperature tuningmeans; tuning the optical resonator cavity containing said pink rubygain element to oscillate at said R2 line at 6924.51 Angstrom wavelengthwith a dispersive tuning means; frequency doubling said R2 line at6924.51 Angstrom to 3462.26 Angstrom; hydrogen Raman down-shifting said3462.26 Angstrom by 8310 cm⁻¹ to the second-Stokes to 4861.342 Angstromto produce said 4861.342 Angstrom hydrogen-beta Fraunhofer lineemission.
 2. A method according to claim 1 further including: cavityalso tuned to said R2 line at 6924.51 Angstrom.
 3. A method according toclaim 2 further including:Q-switching said R2 line at 6924.51 Angstromprior to said frequency doubling.
 4. A method according to claim 3further including:amplifying said R2 line at 6924.51 Angstrom prior tosaid frequency doubling.
 5. A method of emitting at a 4340.50 Angstromhydrogen Fraunhofer line of peak blue-seawater transmission and minimumsolar radiation and high blue-ocean transmission comprising:pumping apink ruby gain element doped with approximately 0.05% chromium ion witha pumping source means to emit at least an R2 line; temperature tuningsaid pink ruby gain element doped with approximately 0.05% chromium ionto approximately +8.13° C. to emit at said R2 line at 6927.00 Angstromwavelength with a temperature tuning means; tuning the optical resonatorcavity containing said pink ruby gain element to oscillate at said R2line at 6927.00 Angstrom wavelength with a dispersive tuning means;frequency doubling said R2 line at 6927.00 Angstrom to 3463.50 Angstrom;methane Raman shifting said 6927.50 Angstrom by 5832 cm⁻¹ to thesecond-Stokes to 4340.50 Angstrom to produce said 4340.50 Angstromhydrogen Fraunhofer line emission.
 6. A method according to claim 5further including:dispersively suppressing an R1 line in a laseroscillator cavity also tuned to said R2 line at 6927.00 Angstrom.
 7. Amethod according to claim 6 further including:Q-switching said R2 lineat 6927.00 Angstrom prior to said frequency doubling.
 8. A methodaccording to claim 7 further including:amplifying said R2 line at6927.00 Angstrom prior to said frequency doubling.